Field of the Invention
This invention relates to a protection system for a capacitor bank for low speed discharge.
Description of the Prior Art
FIG. 1 shows a protection system for a conventional capacitor bank of a large capacity, which is shown, for example, in the paper entitled "Protection System of Capacitor Bank for Low Speed Discharge and Method for Protecting Case Destruction" written by Kunio Kozuno and Kunichi Inoue, Proceedings of Seminar on Static Apparatus, the Institute of Electrical Engineers of Japan, (Dec. 12, 1980). In FIG. 1, reference numerals 1A, 1B, 1C, and 1D each are a capacitor unit, and particularly 1A shows a damaged capacitor. Reference numeral 2 is a coaxial cable for connecting the capacitor units 1A, 1B, 1C, and 1D in parallel with one another, 3 is a short-circuit device which, when a short-circuit (dielectric breakdown) occurs in one among a plurality of capacitor units connected in parallel with one another, short-circuits terminals of the damaged one, 4 is a lead wire, 10 is a switch for main discharge, and 11 is a load.
Next, the operation will be described. At first, in an ordinary capacitor bank for low speed discharge, capacitor units 1A, 1B, 1C, and 1D are connected in parallel with one another by the coaxial cables without using special decoupling elements. Accordingly, when short-circuit (dielectric breakdown) occurs in one capacitor unit 1A, energy stored in the other sound capacitor units concentrically flows into the damaged capacitor unit 1A, and cases and insulators of the capacitor units may be destroyed by an electromagnetic mechanical force acting on the cable due to the concentrated energy, bringing about danger of outflow of oil from the damaged capacitor unit 1A, ignition to oil, further a fire, and so forth.
In order to protect from the danger, it is necessary to electrically isolate the damaged capacitor unit 1A from the other capacitor units 1C and 1D before excessive energy flows into the damaged capacitor unit 1A or to short-circuit the damaged capacitor unit 1A with a low inductance circuit.
FIG. 1 shows an example, wherein the two capacitor units 1A and 1B are connected in parallel with each other, and a terminal of either of the capacitor units is connected via the short-circuit device 3 to the coaxial cable 2 for wiring. The short-circuit device 3 has a lead wire 4 connected thereto, and the other end of the lead wire 4 is connected to terminals on the opposite side of electrodes of the capacitor units 1A and 1B. The short-circuit device 3 comprises, for example, a thin aluminum foil and so forth, and is insulated from the side of the lead wire 4. When the capacitor units 1A and 1B operate normally, they are charged with predetermined energy. But, if short-circuit occurs, for example, in one capacitor unit 1A of the paired capacitor units, a large current flows into the damaged capacitor unit 1A from other capacitor units 1B to 1D. Since the greater part of the current concentrates upon a short-circuited part, the large current flows in a short time. Therefore, the aluminum foil of the short-circuit device 3 is vaporised explosively, breaking the insulating material, and the lead wire 4 on the outside is electrically connected to terminals on one side of the capacitor units 1A and 1B. On the other hand, since a first short-circuit current I.sub.1 flows into the damaged capacitor unit 1A, from the plurality of capacitor units connected in parallel with each other except the capacitor unit 1B paired with the damaged capacitor unit 1A, through the coaxial cable 2 for wiring, a circuit through which the current I.sub.1 flows has a larger than the circuit of 1A and 1B capacitor. Accordingly, the current I.sub.1 flows into later in time than the second short-circuit current I.sub.2 which flows into the damaged capacitor unit 1A from the normal capacitor unit 1B paired with the damaged capacitor unit 1A and operates the short-circuit device 3. As described above, the first short-circuit current I.sub.1 flows into the side of the lead wire 4 and does not flow into the damaged capacitor unit 1A. In other words, the first short-circuit current I.sub.1, the large current is by-passed through the lead wire 4, to protect from a fire due to the damaged capacitor unit 1A.
Since a conventional protection system for a capacitor bank is constituted as described above, upon the occurrence of damage of a capacitor due to dielectric breakdown, an extremely large (a first) short-circuit current flows through the coaxial cable, so that cables, insulators and so forth may be destroyed by electromagnetic mechanical force. There is, therefore, a problem that a capacitor bank having a large capacity can not be constructed.